1. Field of the Invention
The present invention relates to an articulated robot which is used to load/unload a work-piece to/from a cassette. Specifically, the present invention relates to an articulated robot which is capable of performing a stable operation with linear interpolation.
2. Description of Related Art
In order to load/unload a large work-piece such as a semiconductor wafer to/from a cassette, an articulated robot is used in which a plurality of arms is turnably connected with each other to perform extensible operation or the like by transmitting a rotating force of a rotating drive source.
For example, there is a SCARA (Selective Compliance Assembly Robot Arm) type robot as described in Japanese Patent Laid-Open No. 2003-170384. This robot realizes bending and stretching of an arm body (first arm and second arm) by turning the first arm and the second arm in a horizontal plane. Further, loading/unloading of a work-piece is performed by means of that a hand part turnably held at a tip end of the second arm is advanced and retreated to and from the cassette.
According to the SCARA type robot as described in Japanese Patent Laid-Open No. 2003-170384, there is a merit that an installation space for a running axis (rail or the like for running a base in a horizontal direction on which a plurality of arms is installed) is not required, which is different from a case that a cylindrical coordinate type robot is used.
However, in the conventional SCARA type robot, when the first arm and the second arm are controlled with linear interpolation (in other words, when one point on an arm is controlled to be linearly moved), an angular velocity of the second arm is rapidly changed due to overlapping of the two arms and thus carrying of a semiconductor wafer may be unstable. More specifically, it will be described below with reference to FIG. 9.
FIG. 9 is an explanatory view showing states in which the first arm 101 and the second arm 102 are controlled with linear interpolation in a conventional SCARA type robot 100. In FIG. 9(a), the conventional SCARA type robot 100 includes a first arm 101 whose base end part 104 is mounted on a base (not shown), a second arm 102 which is connected with a tip end part of the first arm 101, and a hand part 103 which is connected with a tip end part of the second arm 102. The hand part 103 supports a work-piece (object to be carried) 200.
In order to move the work-piece 200 in an “a” direction (the “X”-axis direction in the X-Y coordinate system), it is assumed that the first arm 101 and the second arm 102 are controlled with linear interpolation (in other words, a case that a tip end part “X” of the second arm 102 is linearly moved in the “a” direction). When the second arm 102 is turned in a counterclockwise direction around the tip end part of the first arm 101 while the first arm 101 is turned in a clockwise direction around the base end portion 104, the work-piece 200 is linearly moved in the “a” direction.
In this robot 100, when an effective length of the first arm 101 is set to be “L101” and an effective length of the second arm 102 is set to be “L102” (see FIG. 9(a)), an attitude in which an angle formed by the first arm 101 and the second arm 102 becomes 0° becomes a singular point. When linear interpolation is performed to pass the vicinity of the singular point, in other words, when “L” shown in FIG. 9(a) traces a trajectory of a value close to (“L101”-“L102”), the first arm 101 and the second arm 102 are turned as shown from FIG. 9(a) to FIG. 9(b). After that, as shown in FIGS. 9(b) and 9(c), the second arm 102 is turned with angular velocity in a clockwise direction around the tip end part of the first arm 101. On the other hand, the first arm 101 is always turned in the clockwise direction around the base end part 104 in FIG. 9(a) through FIG. 9(c).
In FIG. 9, in order to carry the work-piece 200 stably, a carrying speed in the “a” direction of the work-piece 200 is set to be constant (a carrying distance in the “a” direction of the work-piece 200 (moving distance of the tip end part “X” of the second arm 102) from FIG. 9(a) to FIG. 9(b) is set to be substantially the same as that from FIG. 9(b) to FIG. 9(c)). When paying attention to a turning angle of the first arm 101, the turning angle from FIG. 9(a) to FIG. 9(b) is about 90°-θ2 (θ1 is regarded as about 0°) and the turning angle from FIG. 9(b) to FIG. 9(c) is θ2+θ3. In these turning angles, the angle of about 90°-θ2 from FIG. 9(a) to FIG. 9(b) is obviously large. In other words, in order to set a carrying speed in the “a” direction of the work-piece 200 to be constant when the tip end part “X” of the second arm 102 is moved near the singular point, an angular velocity of the first arm 101 is required to be rapidly (at a high speed) changed with the base end part 104 of the first arm as the singular point. This may cause vibration to the first arm 101 and the second arm 102 and thus carrying of the work-piece 200 may be unstable.
Further, in order to rapidly change (at a high speed) the angular velocity of the first arm 101, a remarkably large driving force (torque) is required. However, in order to realize this requirement, an expensive motor is required and, as a result, manufacturing cost may be increased.
With respect to this point, according to the articulated robot disclosed in Japanese Patent Laid-Open No. Hei 11-138474, it can be structured that the singular point is not generated during carrying of the work-piece. In other words, in this articulated robot, a first turning shaft is formed at an eccentric position to the center of rotation of a rotary table to structure the rotary table as a first arm part and an arm which is turnably held around the first turning shaft is structured as a second arm part, a third arm part is turnably held at a tip end of the second arm part, and a hand part is turnably held at a tip end of the third arm part. In addition, the first arm part, the second arm part, the third arm part and the hand part are respectively turned and controlled so as not to generate the singular point. However, this “turning and controlling respectively” becomes a complicated control due to a lot of control parameters, and a great deal of labor is required to prepare the control program.
In view of the problems described above, an object of the present invention is directed to provide an articulated robot which is capable of stably carrying a work-piece with simple control without using an expensive motor generating a large driving force.